1. Field of the Invention
This invention relates to an automatic gain control circuit which, even if the amplitude of an input signal applied thereto is increased to a value greater than a predetermined value, provides a constant output amplitude.
2. Description of the Prior Art
One example of a conventional automatic gain circuit is shown in FIG. 1. In this circuit, an input signal applied to a terminal 1 is applied through a DC stopping capacitor 2 to an amplifier constituted by an npn type transistor 3 and a load resistor 4, as a result of which the input signal is amplified by the amplifier. A DC input bias current to the amplifier is obtained from a DC source 5 and a resistor 6 and is varied by an npn type transistor 7, so that the amplitude of an output signal is maintained constant even if the amplitude of the input signal applied to the terminal 1 is changed. Then, the output signal provided at a terminal 8 connected to the collector of the transistor 3 is amplified, and thereafter rectified, and the resultant signal is applied, as a control signal (or an AGC signal), to the base of the transistor 7 through a terminal 9 connected thereto. If the amplitude of the output signal is increased as the amplitude of the input signal increases, the current flowing in the transistor 7 is increased; while if the amplitude of the input signal is decreased, the current flowing in the transistor 7 is decreased. Thus, as the base bias of the transistor is varied in response to the increment or decrement of the amplitude of the input signal, the above-described circuit can be operated as an automatic gain control circuit. However, the conventional automatic gain control circuit is still disadvantageous in that the distortion of the output signal is increased with the increase in voltage of the control signal applied to the terminal 9 (or the increase in amplitude of the input signal). The reason for this resides in that as the voltage of the control signal is increased, the collector potential of the transistor 7 is decreased, as a result of which the amplification operation of the transistor 3 is shifted from class A to class B, and class B to class C.